Zirconium metallic tubes, such as those formed from Zircaloy-2 or Zircaloy-4, are used in nuclear reactor fuel assemblies to contain the nuclear fuel. Such tubes, or cladding, must meet stringent requirements in order to provide reliability of the fuel elements. It is important, therefore, to ensure that cladding variability be maintained within limits that do not lead to unacceptable failure rates of fuel rods. A goal of zero defects in such cladding thus suggests the need for improved process control and inspection techniques to insure that all cladding in a fuel assembly meet predetermined specifications.
In general, a supply of cladding in a fuel assembly is qualified for use by measurement of tube properties on a limited number of specimens. The destructive nature of the measurements precludes total inspection of the cladding and allows the potential use of a limited number of out-of-specification tubes in a fuel assembly.
Current specifications for Zircaloy fuel cladding can include the measurement of the contractile strain ratio (CSR) which is one measure of the mechanical anisotropy of the cladding material. The CSR measurement is destructive and is derived from strain measurements over a two inch gage length in the sample. Such a local measure of mechanical anisotropy requires the assurance that adequate process control is maintained so that the measured CSR is representative of the entire lot of cladding.
Due to the above limitations of the current specifications for mechanical anisotropy, alternate techniques which are compatible with one hundred percent inspection and are sensitive to cladding anisotropy are desirable. Mechanical anisotropy in cladding is controlled by crystallographic texture and thus, a technique which is sensitive to texture variations is a more direct way of ensuring that mechanical anisotropy requirements are met.
While X-ray diffraction is generally regarded as a laboratory technique, technical advances in X-ray tubes and detectors have enabled X-ray diffraction to be performed in a timely manner outside of the laboratory. This is best illustrated by the measurement of residual stresses by X-ray diffraction in the field by portable, hand-held units. Such measurements can be performed in about ten seconds. As examples of such residual stress measurements by X-ray diffraction, reference is made to U.S. Pat. No. 3,934,138 and U.S. Pat. No. 4,095,103, the contents of both said patents being incorporated by reference herein. In U.S. Pat. No. 3,934,138, an instrument is used that incorporates two separate position-sensitive X-ray detectors. Changes in diffraction angle are measured relative to an unstressed first specimen of the same material as the body whose stress is measured, and the instrument is calibrated using a second specimen subjected to a known stress. In U.S. Pat. No. 4,095,103, a quick and precise measurement of residual stress is effected by performing two diffraction angle measurements successively with only a single detector. The measurement is made by positioning an X-ray source to direct a beam toward the principal surface of the test specimen. The X-rays are diffracted by the specimen and the angle of diffraction is measured with a position-sensitive proportional counter, and diffraction peaks located. The X-ray source and detector are rotated by a predetermined angle and a second diffraction peak located. Residual stress is then calculated by applying formulae derived from linear isotropic theory.
It is an object of the present invention to characterize the crystallographic texture of metallic tubes, such as nuclear fuel cladding, by an X-ray diffraction technique.